With increasing deployment of relatively wideband communications systems, for example CDMA wireless systems with a bandwidth of for example 1.28 MHz, it has been recognized that relatively narrowband interference at frequencies within the wideband spectrum can present a significant problem. For example, such narrowband interference can arise from other communications systems, such as AMPS (Analog Mobile Phone System) communications systems in adjacent areas especially near cell boundaries of the CDMA systems, and narrowband FM (frequency modulation) communications systems, each with a channel bandwidth of for example 30 kHz.
Although attempts are typically made to avoid such interference, for example by allocating frequency spectrum use for different systems in different areas, such interference can still occur and can have serious adverse effects on the performance and operation of the CDMA wireless systems. Similar problems can also arise due to other narrowband interference with CDMA wireless or other wideband communications systems.
International Patent Publication No. WO 00/46929 published Aug. 10, 2000, Lockheed Martin Canada (Charles E. Jagger et al.), entitled “Maintaining Performance Quality Of Broadband System In The Presence Of Narrow Band Interference” describes an adaptive notch filter (ANF) arrangement using one or more notch filters included in a receive signal path of a CDMA wireless system to filter out detected narrowband interference at one or more frequencies.
In this arrangement, an RF signal on the receive path is down-converted using a frequency-controlled local oscillator (LO) signal to an IF (intermediate frequency), notch filtered, and up-converted back to an RF signal, the LO frequency being determined by the narrowband interference frequency to be filtered. The narrowband interference is detected by an FM receiver responsive to the IF signal to produce a RSSI (received signal strength indication) signal for the narrow frequency band, this RSSI signal being compared with a threshold. Instead of providing an FM receiver for each notch filter, a modification of the arrangement uses a scanning FM receiver, and the threshold can be an adaptive threshold determined by adding a constant offset or increment to an average composite power or signal level determined from stored RSSI signals obtained while tuning the FM receiver through the CDMA band.
This arrangement has a disadvantage of involving the complexity and costs of an FM receiver for each notch filter, or a scanning FM receiver for a plurality of notch filters. Also, the arrangement is only effective for FM interference, so that narrowband interference which is not FM is not filtered out. In addition, a bypass switch is required for each notch filter, to bypass the respective notch filter in the event that it is not required to filter out any narrowband interference.
Accordingly, there is a need to provide an improved arrangement and method for detecting and reducing narrowband interference in a wideband signal.